A landmark new study estimates that four million new cases of childhood asthma globally, which is 13% of those diagnosed, could be attributable to air pollution from road transport.

The research published in the Lancet Planetary Health Journal is the first global assessment of the impact of exposure to nitrogen dioxide on childhood asthma.

Lead author Dr Ploy Achakulwisut, from George Washington University, in the US, said: "Our study indicates that policy initiatives to alleviate traffic-related air pollution can lead to improvements in children's health and also reduce greenhouse gas emissions." 

It is thought that pollution from traffic may damage airways, leading to inflammation and the development of asthma in children who are genetically predisposed to the condition.

While it is not clear which pollutant in traffic air pollution is responsible, previous research has suggested exposure to nitrogen dioxide is key.

Traffic emissions can contribute up to 80% of ambient nitrogen dioxide in cities.

The researchers used global data on nitrogen dioxide concentration and asthma incidence to estimate the number of new cases in children aged one to 18 years old which could be related to traffic pollution.

Among the 194 countries assessed, South Korea topped the list, with nearly one third (31%) of new diagnoses linked to nitrogen dioxide exposure.

The researchers said that 92% of cases of childhood asthma attributable to exposure to traffic pollution occurred in areas with average nitrogen dioxide concentrations below the World Health Organisation (WHO) guideline of 21 parts per billion.

"Nitrogen dioxide pollution appears to be a substantial risk factor for childhood asthma incidence in both developed and developing countries, especially in urban areas," senior author Dr Susan Anenberg, also from George Washington University, said.

"Our findings suggest that the WHO guideline for annual average nitrogen dioxide concentrations might need to be revisited, and that traffic emissions should be a target to mitigate exposure."